To purchase a stock or commodity during a price decline. Became popular during the post-housing bubble quantitative easing trend, where stocks were guaranteed to rise until a new dawn of American capitalism magically occurred or, alternatively, the money supply exploded resulting in uncontrollable inflation.

Investor 1: Dude, did you see that silver prices went down? What's with that?
Investor 2: Buy the dip. The Bernank will make sure that all prices are rising.
Investor 1: What?
Investor 2: Buy the fucking dip, you fucking idiot.